1. Field of the Invention
This invention relates to magnetic suspension vibration isolation for gimbal systems used in conjunction with spatial reference or pointing systems. More particularly, it relates to a magnetic suspension which provides a single magnetic gimbal with a six degrees of freedom pointing and bidirectional vibration isolation capability.
2. Background Description
Spacecraft pointing system performance can be significantly affected by disturbances generated by the spacecraft. This is particularly true for systems with very stringent pointing stability requirements; for example, less than one arc second.
To achieve precision pointing stabilities of less than one arc second, it is sometimes required to vibrationally isolate the pointing system from spacecraft generated disturbances and to facilitate the pointing control function in the presence of gimbal structure flexibility. An isolator is required to attenuate vibration over as many as six degrees of freedom; three linear and three rotational axes. Vibration levels are typically of such a magnitude as to require a linear motion capability of at least +0.050 inches in each axis.
One approach has been to utilize a base mounted isolation configuration wherein a pair of pointing system gimbals are mounted between the payload and a vibration isolator. The vibration isolator can comprise either passive or active elements. The passive elements may include, for example, springs and fluid, visco-elastic and magnetic dampers. Active elements may include linearized magnetic actuators and position or velocity sensors with electronic control loops. The gimbals may be supported with either ball bearings or magnetic bearings. The inertial pointing requirements of such a system are shared by the gimbals wherein each gimbal provides freedom only about a single particular axis. The additional compliance represented by this approach may make the gimbal control function more difficult.
Another approach is a payload isolation configuration which utilizes the same gimbal system as the base mounted isolation configuration. The vibration isolator is mounted between the payload and the gimbals. However, the inertial pointing requirements are now shifted from the gimbals to the actuators providing the isolation control. These actuators provide three degrees of pure translational isolation, two degrees of inertial pointing and a third degree of rotational control that may be pure isolation or pointing. The gimbals are controlled to minimize the angular motion between the isolated payload and the gimbal set. The increased control complexity of the payload isolation configuration is offset by a reduction in the pointing control sensitivity to gimbal structural flexibility. Also, the effects of payload mass offset on pointing error can be more precisely controlled in this configuration.
An example of the payload isolation configuration is the well-known Annular Suspension and Pointing System (ASPS), which employes linearized mangetic actuators for isolation and pointing control. This system provides 0.01 arc second pointing stability to payloads while being subjected to spacecraft vibration disturbances. The pointing system gimbals are supported by ball bearings with freedom about only a singular axis for each gimbal. The vibration isolator and vernier pointing system add-ons contain active elements with electronic control loops. The active elements are linearized magnetic actuators and linear position sensors, each with substantial mechanical clearances. These and the electronic control loops are configured to provide six degrees of freedom vibration isolation and vernier pointing capabilities.
A major limitation of these base and payload configurations is that, as add-ons to the gimbal system, they require additional space, weight and power which are all critical factors in spacecraft applications. Even where passive mechanisms are used for the base configuration, there is a volume and weight penalty.
The foregoing illustrates limitations known to exist in present devices. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.